The present invention relates to a method and an apparatus for the manufacture of absorbent cores or pads for absorbent articles such as disposable diapers, sanitary napkins, incontinence pads and the like. More particularly, the present invention relates to an apparatus for forming multi-layered absorbent cores by the air-laying technique.
Multi-layered absorbent cores in accordance with the present invention comprise at least two layers of air-laid fibrous material webs, one of the layers being superimposed on the other and comprising fibers mixed with discrete particles. The fibers of the respective layers can be the same or different and consist of hydrophilic or hydrophobic fiber material or a mixture of both. Additionally, the fibers can comprise a cellulosic or chemical pulp. The discrete particles are provided to enhance the absorbing property and the ability to retain body fluids and wastes, also under pressure, and generally comprise an absorbent gelling material in the form of hydrogel particles. PreferablY, the absorbent gelling material comprises so-called superabsorbent. However, the type of fiber and discrete particles of absorbent gelling material does not form an essential part of the present invention and any combination thereof suitable for the intended purpose of the absorbent core can be selected.
For the purposes of the present invention, a standard absorbent article has at least a dual-layer or triple-layer absorbent core wherein an outer (first) or both outer (first and third) layers are formed by a shaped core component essentially consisting of an air-laid fibrous material web. An inner or middle core (second) substantially uniform mixture of fibrous material and discreted particles of absorbent material. The outer layer or layers generally define the shape of the absorbent core and follow the shape of the absorbent article in which the core is incorporated. The insert or middle core layer can either have the same shape as the outer layer or layers or it can have a smaller shape and be placed at a location where the body fluid or liquid absorption and retaining properties thereof are most desired and beneficial in the absorbent article incorporating it.
An apparatus for the manufacture of multi-layered absorbent pads is known from U.S. Pat. No. 4,598,441. In order to produce a dual-layer absorbent pad, two air-laying apparatus respectively in the form of rotatable drums with perforated forming moulds on their peripheries are provided to form the two fibrous web layers. A first transfer roll is provided between the two air laying drums and picks up a formed first fiber web from the first air-laying apparatus and places this on a second fiber web already formed on the second air-laying drum. This dual-layer absorbent pad is then removed by a further transfer roll from the second air-laying drum and placed on a conveyor for further processing. An optional feature of this known apparatus is the placement of an insert between the two main fiber layers, said insert possibly being a superabsorbent medium. The superabsorbent medium is applied as a discrete layer in powdered or granulated form on the first fiber web while the latter is transferred on the first transfer roll from the first to the second air-laying drum. Suction boxes are provided inside the respective cylinders forming the air-laying drums and the transfer rolls to maintain the fiber layers thereon while they are being transported through the apparatus. However, the air-laying of a fibrous web substantially uniformly mixed with discrete particles of superabsorbent is not disclosed. In any case, this known apparatus is not suitable for producing an absorbent core comprising such a fibrous web uniformly mixed with discrete particles because such particles would fall out along the long path of conveyance through the apparatus. Additionally, the known apparatus is relatively bulky.
Another example of a method and an apparatus for forming-air-laid fibrous absorbent cores is known from FR-A-2 690 843. A first supply of air-entrained fibrous material is deposited on a first movable forming element, and a second supply of air-entrained fibrous material is deposited on a second movable forming element. Subsequently, the first formed component is positioned on an assembly cylinder, and the second formed component is positioned onto the first formed component on the assembly cylinder. In addition, the device comprises a transfer cylinder to transfer the absorbent cores to a conveyor belt. However, the. manner in which superabsorbent material could be included and whether such material was dispersed with the fibrous material or not is not disclosed. Moreover, the apparatus according to FR-A-2 690 843 is relatively bulky as well because of the additional assembly and transfer cylinders.
A method and an apparatus for forming absorbent cores or pads is also known from GB-A-2 124 264. The absorbent core comprises two layers of air-laid fibrous webs which may be of the same or different types. Separate streams of air-entrained fibers are air-laid on an endless perforated belt at locations spaced from each other along the moving belt to form a layered pad. Air from the air-entrained fiber streams is removed via a vacuum chamber disposed beneath the perforated belt.
However, various problems arise in air-laying absorbent cores with a multiplicity of layers including one or more layers containing a mixture of fibers and discrete particles. As the air must be removed from beneath the perforated forming element of the air-laying apparatus, some of the discrete particles mixed with the fibers can be drawn through the perforated forming element so that these particles, which may consist of expensive absorbent gelling materials, are lost. This has the additional disadvantage that the absorbent core is reduced in absorbent capacity. A further problem that can arise is the blocking of air flow through the perforated forming element by larger absorbent particles lodged in the perforations so that the finished absorbent pad does not have the desired structure.
These problems are partially solved by a method and an apparatus disclosed in EP-A-0 292 624 for forming air-laid absorbent cores having a multiplicity of layers. For an absorbent core comprising two main layers, air-entrained fibrous material is split into streams and fed to two separate air-laying apparatus arranged along a conveyor system. In the first drum-type air-laying apparatus for producing the layer comprising a fibrous web mixed with discrete particles of an absorbent gelling material, a so-called dusting layer is first formed by blowing a first air-entrained fiber stream onto the perforated forming element of the air-laying apparatus. A main stream of air-entrained fibrous material mixed with discrete particles is then blown onto the dusting layer to form the main fibrous layer mixed with the discrete particles. The dusting layer acts as a filter screen to prevent passage of small discrete particles through the forming element and prevent large particles from plugging the openings therein.
However, the method and apparatus known from EP-A-0 292 624 have various disadvantages. The final absorbent core is produced from the respective fibrous webs formed in the two air-laying apparatus located at a distance from each other in that the two formed webs are placed on separate endless conveyor belt systems and conveyed from there to a location at which they are only then united to produce the finished absorbent core. Therefore, the speed of the arrangement is limited as too high a speed of the respective conveyers may result in the fibrous webs located thereon being damaged or lifted off the conveyor due to air resistance. Further, the apparatus as a whole takes up a lot of space and requires a complicated and expensive conveying system. A particular disadvantage also lies in that the fibrous web comprising the discrete particles must travel long distances along which particles can fall out of the web so that the final absorbent core will have a reduced absorbent capacity. Although this fibrous web mixed with particles can be enwrapped in tissue or the like before being united with the other fibrous web, this process introduces increased complications and costs. Further, the absorbent properties of the final product may be adversely affected by the presence of the tissue and such tissue hinders the possibility of at least loosely connecting the respective fiber webs to each other to increase the strength of the final product. Finally, the known apparatus does not permit easy uniting of the respective layers in terms of accurate targeting or placement of the fiber and particle web on the second fiber web to form the finished absorbent core.
It is the object of the present invention to provide an efficient method and apparatus for forming improved air-laid fibrous absorbent cores having at least two layers with at least one layer having fibers with discrete particles dispersed therein.
This object is solved in accordance with the present invention by an apparatus comprising the features of claim 1 and a method. comprising the features of claim 24. Particularly advantageous embodiments of the inventive apparatus and method are described in the dependent apparatus and method claims.
According to the present invention, the apparatus and method provide two separate air-laying means having movable perforated forming elements thereon. The air-laid components making up the absorbent core are separately formed by applying a stream of air-entrained fibrous material or, as applicable, a stream of a mixture of air-entrained fibrous material and discrete particles of absorbent material onto the respective forming element of the respective air-laying means. Thus, the (second) component of the absorbent core formed of the mixture of preferably substantially uniformly mixed fibrous material and discrete particles is formed in a separate (second) air-laying means independently of other (first and possibly third) components of the final absorbent core which are produced on the other (first) air-laying means. This allows the production and processing parameters for forming the mixed second component to be set and adjusted as desired independently of the formation of the other absorbent core components. This avoids having to take disadvantageous limitations into account as, for example, if the mixed second component is formed on the same air-laying means as the other components of the absorbent core.
In particular, the design of the second air-laying means for the mixed second component can be different to the design of the air-laying means for the other components. This especially includes the design of the (second) forming element of the second air-laying means, the supply of the fibrous material and discrete particles thereto and the formation conditions. Thus, the mixed second component of the absorbent core formed on the second air-laying means can have any desirable different size, shape or composition as compared to the other component or components of the absorbent core formed on the (first) forming element of the first air-laying means. Additionally, as compared to the first forming element in the first air-laying means, different perforation sizes in the second and different differential air pressures across said second element can be used to produce the mixed second component.
A particularly important advantage of the inventive apparatus and method results from the transfer of the mixed second component from the second air-laying means onto a first component already formed in the first forming element of the first air-laying means while the first component is still located in the first air-laying means. This provides the particular benefit of simplifying the targeting of the mixed component on the other layer (first component). Further, undesirable excessive mixing of discrete particles in the mixed second component into the underlying first component is substantially avoided as compared to a known method in which the mixed second component is formed directly on the first component already. formed in the same air-laying means.
In this disadvantageous method described above, the second component of mixed fibrous material and discrete particles is applied under pressure onto the first component. Further, an underpressure is possibly also created beneath the formed first component to draw the fibrous material and discrete particles onto this layer to form the mixed second component thereon. This causes disadvantageous migration of particles from the mixed second component layer into the underlying first component layer. The first component layer is usually employed as a so-called acquisition layer in the absorbent core of a final absorbent article such as a disposable diaper and is therefore the first layer of the absorbent core which receives body fluid or waste. If discrete particles of, for example, an absorbent gelling material are present in such an acquisition layer, this can partly lead to rewet or slow the acquisition rate of the body fluid. The discrete particles should preferably only be present in the mixed component layer of the absorbent core as these particles, in the form of an absorbent gelling material, have the primary function of retaining liquid, also under pressure to avoid rewet, at a distance from the location of acquisition where quick removal of fluid from the outer surface of the article is desired. Additionally, absorbent gelling material in the acquisition layer can also lead to so-called gel-blocking of body fluid penetrating through the acquisition layer since swollen gelling material hinders the throughflow of the body fluid.
The present invention avoids all of these problems by forming the mixed second component separately from the first before uniting these. Also, the present invention does this in a particularly advantageous manner by also effectively reducing the loss of expensive discrete particles comprising, for example, absorbent gelling material from the finished absorbent core on account of transferring the second component onto the first component while the latter is still on the first forming element of the first airlaying means. Additionally, transfer of the mixed second component onto the first component while this is still on the first air-laying means permits some interconnection of the components, especially if a third component is subsequently formed on top of the first and second components as in a preferred embodiment of the present invention.
The transfer of the mixed second component onto the first component while this is still on the first air-laying means also contributes to reducing the size and cost of the apparatus and removes the need for a complicated means to unite the components.
In preventing the loss of discrete particles out of the absorbent core after the mixed second component is transferred in the inventive manner onto the formed first component and out of the apparatus as a whole, other problems are also prevented. For example, in an absorbent core for a disposable diaper or the like, the production rate is approximately 600 cores per minute, which corresponds to 10 cores per second, If a core has an average discrete particle weight of 10 g, this implies that the process consumes approximately 100 g per second. If even only approximately 5% of the discrete particles is lost out of each absorbent core during manufacture, then a considerable quantity of-expensive absorbent material is lost over time. Further, these large quantities of particles have to be removed from the apparatus to maintain proper operation thereof. The reduction of the loss of discrete particles according to the present invention overcomes these problems.
Further, as already described, the present invention relates to an absorbent core having a second component comprising a mixture of fibrous material and discrete particles, preferably substantially uniformly mixed. Although it is known in the art to provide absorbent cores with a discrete layer of particles, such a core is disadvantageous in terms of the final performance under wetting of the absorbent core. More importantly, however, in manufacture, the discrete particles are more likely to be lost because they are not dispersed within a fiber web. These problems are also avoided with the present invention.
In an advantageous preferred embodiment of the present invention, a third component of the absorbent core comprising air-laid fibrous material is formed on the united first and second components. This contributes to further reducing. the loss of discrete particles from the absorbent core during manufacture. Preferably, the third component is formed while the first and second components are still positioned on the first forming element of the first air-laying means to prevent loss of any particles as soon as possible and to also reduce the size of the apparatus. Additionally, during formation of the third component, the connection between the three components is strengthened by fibers of each component mingling with each other.
Preferably, the supply of air-entrained fibrous material for forming the first and third components is produced by connecting a source of fibrous material to an air fan which entrains the fibrous material and supplies this to the first air-laying means. Various conventional means may be used as the source of fibrous material. For example, the fibrous material can be produced from fiber sheets passed through a rotary disintegrator and further to the air fan.
Advantageously, the supply of air-entrained fibrous material and discrete particles includes a similar conventional source of fibrous material and any convenient source of discrete particles such as a hopper. Preferably, the fibrous material and discrete particles can be mixed most advantageously in an air fan before being supplied to the air-laying means so that a substantially uniform mixture or dispersion of the particles in a fiber web is produced as the second fibrous component on the second forming element. Even better mixing is achieved by introducing the fibrous material and discrete particles into a duct (104, 204) upstream of the air fan as fiber flocks may otherwise be formed in the fan which no longer mix with the particles.
In accordance with a further advantageous embodiment of the invention, at least one or both of the forming elements of the respective air-laying means comprise one or more cavities or moulds which define the shape of the absorbent core component to be produced. The base of each cavity comprises the perforations of the forming elements upon which the respective fibrous web is formed and trough which the air passes to be separated from the entrained material for forming the component. Further, the cavities can be exchangeable so as to be able to provide different shapes of the components. For example, the first (and possibly third) fibrous component formed on the first air-laying means can have cavities in the shape of an hour glass so as to produce the commonly known shape of a disposable diaper. However, in accordance with the invention, as the second component comprising the mixture of fibrous and discrete particles is formed separately from the first and third components in the separate second air-laying means, depending on the desired design of the final product, the cavities forming the second forming element can be the same size or can be smaller than the cavities of the first forming element when the entire surface of the unfolded diaper comprising. the absorbent core does not require total surface coverage by the second component. This is particularly advantageous in that areas of the mixed second component as compared to the area of the unfolded diaper as a whole can be omitted to avoid material wastage and therefore saves expensive absorbent (gelling) material and fibers. In particular, as viewed in the unfolded state, an absorbent article such as a disposable diaper does not require an absorbent core with a mixed second component of fibrous and discrete particles which extends up to the longitudinal extremities of the article. This is because the fluid wicking effect of the article towards the longitudinal extremities is normally relatively poor and the absorbent material and fibers are not or barely wetted in these areas. The same applies to a lesser extent in respect of the extremities of the article as seen in the direction transversely to the longitudinal extension, i.e. the width of the article. The possibility of accurately positioning a smaller mixed second component in accordance with the present invention while simultaneously preventing loss of discrete particles as described above and further below is therefore a particular advantage of the present invention over prior art techniques for producing absorbent cores with a component of uniforrmly mixed fibers and discrete particles.
The inventive forming of the mixed second component of the absorbent core on a separate second air-laying means also produces advantages in the manufacturing process as compared to previous systems in which a narrower mixed second component is formed in the same air-laying means directly on the first component, which covers the entire width of the forming element.
In the previous system referred to above, a forming duct is provided above the forming element of the air-laying means, the duct having a narrower width than the forming element and the first component which is already present therein. By appropriately positioning the forming duct for the air-entrained mixture of fibers and discrete particles to guide the mixture onto the forming element of the first component, the narrower mixed second component is formed at the desired location. However, as the forming duct for the second component is narrower than the perforated forming element therebeneath, a significant amount of air can escape between the end of the narrower duct and the wider forming element. Air for entraining the fiber and discrete particle mixture is expensive to produce, transport and filter. Further, it is not possible in this type of apparatus to prevent this air loss by closing the exposed area between the narrower forming duct and the wider forming element because this would otherwise result in the second component being formed across the entire width of the forming element, which wastes fiber and discrete particles. Therefore, in a preferred embodiment of the present invention significant advantages in the manufacturing process of the absorbent core are achieved by using a forming duct which has substantially the same width as a narrower cavity in the second forming element of the separate (second) air-laying means because the air loss is substantially prevented.
An additional advantage is provided according to the inventive method and apparatus when using a narrow cavity for the second component. When the second component is transferred and placed on the first component, discrete particles falling out of the sides of the second component are immediately caught by the underlying first component. Therefore, the amount of discrete particles lost in the apparatus is further reduced with little loss of particles into the first component because these are mostly retained by the fibers of the second component.
In a similar manner as described above, forming ducts can also be provided to form the first and third components of air-entrained fibrous material on the first air-laying means and these ducts can have substantially the same width as the cavities in the first forming element so as to prevent loss of air.
The formation the respective components of the absorbent core in the method and apparatus of the present invention can be advantageously controlled by appropriate selection of the differential air pressures across the forming elements and the size of the perforations in the forming elements. The perforations in the first forming element upon which the first and third components are formed should be of a suitable size which represents a compromise between the necessity to provide the air with the easiest possible passage through the forming elements, this advantageously reducing the differential pressure that has to be created across the forming elements, and the need to prevent passage of the air-entrained fibrous material or discrete particles through the forming elements. Therefore, in accordance with the invention, the perforations in the first forming element preferably have a largest dimension perpendicular to the air flow of approximately 1 mm and, more preferably, 0.5 mm. Most preferably, the largest dimension is no larger than approximately 0.2 mm.
On the other hand, as the. mixed second component comprising fibrous material and discrete particles is formed on a separate second forming element in the second air-laying means and air does not have to be drawn through more than one fibrous component, the perforation dimensions in the second forming element advantageously can be smaller than those in the first forming element and the largest dimension is preferably approximately 0.5 mm and, more preferably, 0.2 mm Most preferably, however, the largest dimension is approximately 0.05 mm so as to provide an effective barrier against throughpassage of the discrete particles of, for example, absorbent gelling material such as superabsorbent. Superabsorbents used in absorbent articles normally have a mean average diameter of approximately 0.2 to 0.8 mm so that their passage through the second forming element is effectively reduced or even excluded in the preferred inventive embodiment of the second forming element having its largest perforation dimensions as defined above. Naturally, the present invention is not limited to substantially spherical discrete particles. These can also be of any other regular or irregular shape including oblong or fiber-like shapes.
Preferably, the perforations in the first and second forming elements are formed by a screen such as a wire mesh screen. If required for reasons of strength, the screens can be formed of multiple layers having an increasing mesh diameter in the downstream layers as seen in the direction of air flow, or appropriate support bars can be provided beneath the screen. A combination of layers of mesh screens and support bars is also possible. Naturally, however, the support arrangement must be chosen so as to avoid blocking the air flow as much as possible.
The differential pressure across the forming elements is preferably produced in a known manner by creating a vacuum or underpressure at the downstream side of the forming elements. This underpressure generally improves formation of the fibrous web on the forming element and temporarily maintains the formed web in position until it is to be removed, for example by removing the underpressure and/or creating a differential pressure in the opposite direction to that of the original air flow created during formation and/or using gravitational force. Any means such as a vacuum source in the form of a suction box or the like can be used to create the differential pressures across the forming elements.
The pressures across the first forming element for forming the first (and third) components is approximately 7500 to 9000 Pa, preferably 8500 Pa. However, in accordance with the present invention, the differential pressure for forming the mixed second component of fibrous material and discrete particles in the separate second air-laying means can be considerably higher and is preferably approximately 30000 Pa. This increased pressure provides advantages both in terms of the speed at which the second component can be formed and the final structure of the formed second component.
In a particularly advantageous embodiment of the invention, the second air-laying means is positioned with respect to the first air-laying means such that the mixed second component is transferred directly out of the second forming element onto a formed first component on the first forming element so that there is a minimum distance of transfer and therefore a minimum loss of discrete particles out of the mixed second component during the transfer operation.
In an alternative advantageous embodiment of the present invention to that described above, a transfer means is provided between the first and second air-laying means to transfer the second component onto a formed first component in the first air-laying means. The inventive transfer means comprises at least one pick-up means to pick up the second component and transfer it onto the first component. Even though the transfer path is somewhat longer than the arrangement for directly transferring the second component as described above, the transfer means also provides significant advantages. Preferably, there is a plurality of pick-up means which are each rotatable around the periphery of the transfer means at a variable orbital distance from the axis of rotation of the pick-up means. This provides an improved and positive transfer or delivery of the mixed second component onto a formed first component if the latter lies below the upper surface of a recess or cavity in the first forming element. Consequently, the best possible localisation or targeting of the second component with respect to the first (and subsequently laid third) component in the absorbent core is achieved.
Another advantageous embodiment of the present invention comprises first and second air-laying means with rotatable forming drums on which the forming elements are formed around the periphery of the drums. When the forming elements comprise a plurality of cavities, the rotating drums allow the production of the absorbent cores at a high rate in a relatively small space. Further, the supply of the materials to form the various components can be arranged around the peripheries of the drums to save further space as compared to known arrangements of air-laying means and supplies of material along a straight production path. As already stated previously, the cavities of the respective forming elements can be exchangeable. Therefore, the shape and dimensions of the first, second and third components can be changed as desired by simply exchanging the cavities of the respective forming elements.
In a most preferred embodiment of the present invention, the second forming drum is provided above the first forming drum to achieve a maximum space saving in terms of the length of the production line for an absorbent article such as a disposable diaper or the like. Furthermore, this superimposed relationship of the two drums allows for simple transfer of the second component onto the first component either directly or by way of the transfer means as previously described. Naturally, in this embodiment it is possible to transfer using only the influence of gravity. However, for high production rates and positive targeting or delivery of the second component onto the first component, it is preferable to use vacuum means such as suction boxes or the like within the forming drums for creating differential pressures between the first and second forming elements . so as to consistently accurately transfer the second component onto the first component Alternatively, the transfer means described above can be used to achieve such an accurate transfer.
In another advantageous embodiment of the present invention, the finished absorbent core formed in the first and second air-laying means is received from the first air-laying means by a conveying means or any other means and removed for further processing.